In current fuel injection systems of the common rail type, a low-pressure pump supplies the fuel from a tank to a high-pressure pump, which in turn supplies the fuel to a common channel (known in jargon as a “common rail”). The common channel is connected to a series of injectors (one for each cylinder of the engine), which are actuated cyclically in order to inject part of the fuel under pressure present in the common channel into a respective cylinder. In order for the injection system to operate correctly it is important that the value for the pressure of the fuel inside the common channel is kept, moment by moment, equal to a desired value that generally varies over time; for this purpose, the high-pressure pump has dimensions for supplying the common channel, in all operating conditions, with an amount of fuel exceeding the actual consumption and the common channel is coupled to a pressure regulator that keeps the value for the pressure of the fuel inside the common channel equal to a desired value that generally varies over time, discharging the excess fuel to a recirculation channel that reintroduces said excess fuel upstream of the low-pressure pump.
Known injection systems of the type described above have various disadvantages, in that the high-pressure pump must have dimensions for supplying the common channel with an amount of fuel slightly exceeding the maximum possible consumption; however, this condition of maximum possible consumption proves fairly rare and in all other operating conditions the amount of fuel supplied to the common channel is much greater than the actual consumption and therefore a significant proportion of this fuel has to be discharged by the pressure regulator into the recirculation channel. Clearly, the work performed by the high-pressure pump in order to pump fuel that is subsequently discharged by the pressure regulator is “useless” work, therefore known injection systems have very low energy efficiency. Moreover, known injection systems tend to overheat the fuel, in that when the excess fuel is discharged by the pressure regulator into the recirculation channel, said bars) to substantially ambient pressure and because of this jump in pressure its temperature tends to increase.
Finally, known injection systems of the type described above are relatively expensive and cumbersome because of the presence of the pressure regulator.
In order to resolve the problems described above at least partly, it has been proposed to use a high-pressure pump with more cylinders, provided with a regulation device capable of excluding one or more cylinders as a function of the engine point so as to reduce the amount of excess fuel. However, this solution proves complicated and expensive and is only able to resolve some of the problems of energy consumption and overheating connected with the presence of excess fuel.
Another solution to the problems described above has been proposed by patent application EP-0481964-A1, which describes the use of a high-pressure pump provided with an electromagnetic actuator capable of varying the flow rate of said pump, moment by moment; however, the methods for controlling the flow rate of the high-pressure pump proposed by patent application EP-0 481 964-A1 are not able to guarantee optimal operation of the injection system in every operating condition.
Moreover, the high-pressure pump proposed by EP-0481964-A1 is structurally complicated and expensive; therefore patent U.S. Pat. No. 6,116,870-A1 proposes another embodiment of a high-pressure pump with variable flow rate. In particular, the high-pressure pump described by U.S. Pat. No. 6,116,870-A1 comprises a cylinder provided with a piston having alternating motion inside the cylinder, an intake channel, a discharge channel connected to the common channel, an intake valve capable of allowing a flow of fuel to pass into the cylinder, a single-direction delivery valve coupled to the discharge channel and capable of allowing a flow of fuel only out of the cylinder, and a regulation device coupled to the intake valve in order to keep the intake valve open when the piston is in a compression phase and therefore to permit a flow of fuel out of the cylinder through the intake valve; the intake valve comprises a valve body moveable along the intake channel and a valve seat, which is capable of being acted upon in a fluid-tight manner by the valve body and is arranged at the end of the intake channel opposite the end communicating with the cylinder; and the regulation device comprises a control member, which is coupled to the valve body and is moveable between a passive position, in which it allows the valve body to act in a fluid-tight manner on the valve seat, and an active position, in which it does not allow the valve body to act in a fluid-tight manner on the valve seat, and an electromagnetic actuator, which is coupled to the control member in order to move the control member between the passive position and the active position.
However, the high-pressure pump proposed by patent U.S. Pat. No. 6,116,870-A1 also has some disadvantages, particularly owing to the cost and electric power consumption of the electromagnetic actuator coupled to the control member.
EP-1188919-A1 discloses a fuel supply system for a direct injection engine which has a variable capacity single cylinder plunger pump and two fuel rails. There are disposed orifices at the upstream side inlets of the both fuel rails, respectively; at the opposite sides to the inlet sides, the fuel rails are interconnected with each other by a connecting pipe. By the fuel supply system, it is capable of increasing a characteristic frequency of the fuel columns, and of stabilizing, suppressing, and smoothing out pressure pulsation in the fuel rails, thereby reducing uneven fuel injections into the cylinders; the system may have a cam which drives a plunger of a high pressure fuel pump to reciprocate once for every two combustion in two engine cylinders.
EP-1162365-A1 discloses a high-pressure fuel feed pump for an internal combustion engine; an intake valve automatically opened and closed by pressure of a pressuring chamber is provided in a fuel intake passage, the intake valve is pushed to open by a plunger of an electromagnetic plunger mechanism, pulling-in operating timing of the plunger is controlled according to the operating condition of an internal combustion engine, and opening time of the intake valve during compression stroke of a pump is controlled to make discharge flow-rate of high pressure fuel variable.
EP-0979940-A1 discloses a device for controlling fuel injection into an internal combustion engine is disclosed; the device comprises an accumulator for supplying pressurized fuel to a fuel injection valve, a high-pressure pump for discharging fuel into the accumulator using the engine as a power source, and a low-pressure pump for discharging fuel into the high-pressure pump using a power source other than the engine. At the start of the engine, the fuel discharged from the low-pressure pump is substantially directly introduced into the accumulator through a pump chamber of the high-pressure pump; to elevate the pressure within the accumulator for a short period to a fuel pressure capable of injecting fuel at the start of the engine, an opening and closing valve is provided in a suction passage that communicates the discharge side of the low-pressure pump with the suction side of the high-pressure pump, and is maintained to be opened at the start of the engine.